Semiconductor wafers and/or devices are traditionally cleaned to prevent as much as possible contamination by foreign particles. Wafer cleaning methods however have their limits, and may not stop diffusion of contaminants into the wafer. For example, contaminant foreign atoms, e.g. foreign contaminant metals, may diffuse, from a wafer front side and/or wafer back side into the wafer. These foreign atoms may be from solder layers formed over the back side of the wafer or back side metallization layers or even direct copper bonding substrates which may be contacted with the semiconductor back side. Foreign atoms from the front side may come from front side metallization layers, e.g. wafer front side interconnects.
Semiconductors, e.g. silicon wafers, during their manufacture, e.g. by Czochralski growth, may incorporate an intrinsic gettering region, rich in oxygen precipitates. Furthermore, alternatively or in addition to wafer cleaning and/or intrinsic gettering, an extrinsic gettering region may be formed. For example, a polysilicon layer may be applied over the wafer back side. Polysilicon may serve as a gettering site for undesired heavy metals.
The polysilicon layer may however be prone to oxidation, for example, at least partial oxidation during processing of the wafer, e.g. during front end processing of the wafer, and before thinning the wafer. Furthermore, the polysilicon layer may traditionally be completely removed during the wafer thinning processing. In addition, for large diameter wafers it may be difficult to obtain suitable polysilicon layers from basic material suppliers.